Fix endless while loop on EOF for pty process

[jthacker]

os.read spec says that when EOF is reached by the fd, an empty bytes
object is returned.

[bugy]

Hi @jthacker , thanks for the PR! Looks like a critical bug. but I'm curisous why nobody saw this issue before. Let me play a little bit with a current code to check why it's working (or if I'm missing something).
After that I'll merge your PR.

[jthacker]

@bugy I am running this on OSX, which might play a part there.

[bugy]

The reason why it's working on linux: non-blocking read should fail with an error if there is no data to read: example documentation
For OSX it should be the same behaviour (according to this link, O_NONBLOCK description)

For that reason there is a code in infinite loop:

except BlockingIOError:
  break

Merging your PR should be fine, to be on safer side. But I still need to find out the reason why BlockingIOError (or any other error) is not thrown. It could be, for example, that non_blocking flag is not set at all, which will influence performance and can cause some other issues

Anyway, thank you for investigating and fixing the issue!